This invention relates to a slurry polymerization of ethylene.
Chromium catalysts have long been known in the art. Supported chromium catalysts have been a dominate factor in the commercial production of polyethylene. Originally, commercial processes used solution polymerization techniques to produce polyolefins. However, it later became evident that a slurry polymerization process was a more economical route to produce commerical grades and quantities of polymer.
Slurry polymerization is unlike solution polymerization, in that, the polymer, as it is formed during the slurry process, is largely insoluble in the inert carrying diluent. This makes it easier to recover the polymer. However, certain control techniques which are easily carried out in a solution polymerization become nearly impossible in a slurry polymerization. For example, in solution polymerization to make a polymer that has a lower molecular weight, as well as higher melt flow index, the temperature can be raised, thus accomplishing this desired result. However, in a slurry polymerization there is a practical limit on increasing temperature because the point is quickly reached where the polymer goes into solution and fouls the reactor.
In general, reactor fouling occurs when polymer particles, in a slurry polymerization process, dissolve into the reactor's liquid phase. Specifically, the polymer particles, while dissolving, increase the volume that they occupy by several orders of magnitude. This increase in volume causes the viscosity to increase in the liquid phase of the reactor. If the polymer particles continue to dissolve, thereby increasing the viscosity, eventually a gel-like substance is formed which plugs the reactor. Unplugging a fouled reactor is a time-intensive and costly undertaking.
Over the years, those in the art have come up with various relationships to help reactor operators avoid a reactor fouling incident. One of the oldest relationships, in this art, is the density v.s. temperature foul curve. (See the FIGURE). For many years, this relationship has been considered to be practically inviolable. This is in spite of the fact that it would be highly desirable to operate a slurry polymerization process at a reactor temperature higher than the foul curve temperature for a specific copolymer density.